The field ion microscope (FIM) was introduced by Muller in 1951. The specimen in this instrument is the anode and an imaging gas (hydrogen, or an inert gas) is introduced into the system at low pressure (10.sup.-3 Pa). Ionized gas species produce the image at the screen. In 1967, Muller, et al. introduced the atom probe field ion microscope (APFIM). In this instrument, the imaging gas is removed and, with the tip operating as an anode, the field is increased to the point where the atoms on the surface are evaporated by the field. A very high electric field (about 10.sup.10 V/m) is created at the surface of the tip by applying a high voltage between the tip and an image screen. The tip is sharply pointed with a radius of curvature at the apex of about 10 to 200 nm. Since the process of field evaporation involves ionizing the atoms, they are accelerated to the imaging screen by the applied field. In the APFIM, the flight times of ions from the specimen to the image screen are used to identify the ions. The field evaporation must be pulsed so that a definite time of departure can be determined. The standing high voltage is kept low enough so that the evaporation rate between pulses is negligible. A very short duration (less than 10 ns), high voltage (20% greater than the standing voltage) pulse is applied which causes evaporation of a fraction of a monolayer of atoms from the specimen surface on each pulse. This approach is known as voltage pulsing or field pulsing.
In the conventional APFIM, an aperture with a virtual size of 0.2-10 nm is placed in the image screen to allow the ions from a specific location on the tip to pass through into a time-of-flight mass spectrometer. Because the identification of elements is based on time-of-flight, all isotopes are detected, and there are no mass limitations either at high or low masses. However, the virtual image is typically only a few atom diameters wide at the detector plane, and no positional information about the ions is recorded. Thus, data from this conventional instrument is inherently one dimensional. The lateral resolution varies from about 0.2 nm to 10 nm depending on the magnification, but the depth resolution is typically the atomic plane spacing in the axial direction of the specimen. Unfortunately, the data collection method makes inefficient use of the sample since most of the specimen atoms that are evaporated are never analyzed.
The imaging atom probe and the wide angle atom probe are early types of three-dimensional atom probes (3DAP) which were developed in response to such inefficiencies. The 3DAP can, in principal, determine the identity and original location of every atom which hits the image screen. It has the ability to measure both the time-of-flight of a given ion to the wide angle image plane and the arrival position on that plane. In this way, both the position and identity of the atoms on the surface of the tip are determined. An initial instrument of this type is discussed in the paper by A. Cerezo, et al., Rev. Sci. Instrum., Vol. 59, 1988, pp. 862, et seq. Cerezo, et al. refer to the instrument as a position-sensitive atom probe (POSAP). Incoming ions strike a microchannel plate and the electron charge cloud that is produced is accelerated onto a position-sensitive anode. The three-dimensional position of an atom is determined from the two-dimensional arrival position on the image screen and by tracking of the arrival sequence of atoms in time as each layer is evaporated. Since the information is electronically recorded, images may be viewed and quantitatively analyzed in a wide variety of formats. A further development of a high-repetition-rate position-sensitive atom probe is shown in U.S. Pat. No. 5,061,850 to Kelly, et al.
Conventional voltage pulsing in 3DAP creates a problem with mass resolution. The applied voltage pulses must have a sub-nanosecond rise time and only a few nanoseconds duration. The ions typically leave the tip on the leading edge of the pulse, close to the maximum voltage. However, they may leave over a range of voltages and thus acquire a range of total energies. The uncertainty in the evaporation voltage of the ions for such a pulse is typically 1-5%. Since potential energy is converted to kinetic energy during flight, this range of voltages leads to a range of flight times for a given mass-to-charge ratio. Thus, the timing resolution or mass resolution suffers in voltage-pulsed atom probes. In short flight path instruments such as a 3DAP this resolution can be as poor as one part in fifty. One part in two hundred is commonly considered to be adequate mass resolution for a large fraction of materials analyses. Better resolution is always beneficial for separating closely spaced isotopic peaks.
Conventional atom probes may be provided with devices which compensate for the kinetic energy spread of the ions in voltage pulsing. The resulting mass resolution can be as high as one part in two thousand. However, there has been no previously demonstrated approach that achieves high mass resolution in a 3DAP without unduly sacrificing image size. The best reported mass resolution in a 3DAP is one part in three hundred utilizing an instrument which has a one meter long flight path but a field of view of only 10 nm laterally. A. Bostel, et al., J. de Physique, Vol. 50-C8 1988, pp. 501 et seq.
The conventional device used to produce voltage pulses with the required performance for conventional atom probes is a mercury-wetted reed switch. Reed switches are mechanical devices, however, and cannot be operated at frequencies much higher than about 200 pulses per second. Since ion flight times do not normally exceed 10 microseconds (1 microsecond in 3DAPs) atom probes could be operated at much higher speeds if higher repetition rate pulsers were available.
It is also possible to momentarily increase the field evaporation rate by pulsing the temperature of the tip, which is known as "thermal pulsing". The use of a pulsed laser for inducing field evaporation is reported in the articles by T. T. Tsong, Surf. Sci., Vol. 70, 1978, pp. 219 et seq., and G. L. Kellogg, et al., J. Appl. Phys., Vol. 51, 1980, pp. 1184 et seq. The major advantage of this approach over voltage pulsing is that the applied field is constant during the pulse, which effectively eliminates this contribution to uncertainty in mass resolution. The use of an electron-beam pulse to stimulate evaporation of ions is also discussed in the aforesaid patent by Kelly, et al., U.S. Pat. No. 5,061,850. In such devices, the electron-beam pulse may heat a small enough volume of a tip (less than 200 nm length) so that the thermal pulse will be very short (sub-nanosecond) and may be repeated at high rates (greater than 106 pulses per second).
Another limitation of atom probes is the preparation of samples into the required geometrical form. Fabrication of a specimen from a metal wire or bulk sample, although tedious, is relatively easy and similar to sample preparation for transmission electron microscopy. However, making a sample with the desired orientation from a multilayer sample or from a semiconductor wafer can be quite complicated and often may be impossible. A flat-plate geometry that can be more easily utilized in chemical analysis instrumentation has recently been proposed for use in a "scanning atom probe". See 0. Nishikawa, et al., Applied Surface Science, Vol. 76/77, 1994, pp. 424-430. This article suggests mechanically microgrooving a flat-plate to produce a specimen with many microtips normal to the plate. This type of specimen would allow selection of analysis areas on various structures not easily investigated with conventional atom probes. It would also make it possible to produce large numbers of separate tips in a single preparation procedure. Preparation of specimens using the technique of "ion beam mask etching" has been done initially by J. A. Liddle, et al., J. de Physique, 49-C6 (1988) 509 et seq. This process produces a random or mosaic array of microtips by deposition of small particles on a substrate with subsequent ion sputtering to form cones of unetched material under the particles deposited on the substrate. Many variations on this approach are possible. Particles of various types including polymer spheres, metallic spheres and ceramic particles can be used as masks.
As discussed above, the scanning atom probe was proposed to reduce atom probe specimen preparation difficulties. However, if the voltage is applied solely between the sample and the detector, evaporation from many tips will occur, leading to superimposition of the data. To overcome this limitation, Nishikawa, et al., supra, proposed that a local-extraction electrode be used to confine the high field to a particular tip. The calculations performed by Nishikawa, et al. demonstrate the feasibility of this concept for a high potential applied between the local electrode and the tip which serves to evaporate the ions and accelerate them.
The concept of a local-extraction electrode is also the basis for the operation of field emitter arrays which achieve field electron emission at very low voltages. Field electron emission currents of 50 to 150 .mu.A can be achieved from single tips of a radius of about 50 nm using 100 to 300 volts. C. A. Spindt, et al., J. Appl. Phys., Vol. 47, 1976, pp. 5248, et seq. This voltage is approximately a factor of 10 less than what is normally required for a conventional field electron emitter with the anode essentially at infinity. Spindt, et al. also concluded that when the tip radius is in the 50 to 150 nm range, rather than an inverse relation between tip radius and field, the radius appears to have only a second order effect on the field. Experiments have also shown field ionization using an extraction-electrode geometry at voltages on the order of 1,000 volts. C. A. Spindt, Surf. Sci., Vol. 266, 1991, pp. 145 et seq. This value is again about a factor of 10 less than what is normally required for field ionization.